An in vitro slice preparation of the hippocampus from guinea pig brain was utilized to study quantitatively the actions of ethanol, benzodiazepines and barbiturates on synaptic inhibitation. Diazepam, in a concentration of 20 uM, was foun to potentiate synaptic inhibition. The separate actions of barbiturates and ethanol, and the interactions of ethanol with benzodiazepines and barbiturates will be examined.